The overall goal of this study is to develop a testing instrument that will permit reproductable assessment of the ability to convert ammonia to urea. Such a test would be used in the future to quantitate the response of individuals with ornithine transcarbamylase deficiency to gene therapy. The specific aims of the project are to 1) develop an oral loading test that will measure the in vivo conversion of 15NH3 to [15N] urea and 2) to determine whether this test will discriminate adult carriers for ornithine transcarbamylase deficiency from a control population.